* Force png tile generation for static maps
If the user tries to generate a static map requesting JPG it will fail
because is going to try to generate the tiles using JPG as format which
is not supported by now, this bug was introduced in the version 4.0.1
So we now force, again, the tiles to be generated as PNG but we pass
the requested format, JPG, to windshaft to generate the final image as
the user reqests
* Added support to define image format in the image assertions
* Added test for JPEG static image generation
Also I've added support for:
- JPEG images
- Different tolerance based on the file type, it seems that due to
different compression we need different tolerance for JPG images
